Define `wk_set_crs()` and `wk_is_geodesic()` for s2_geography class

[paleolimbot]

To avoid errors like this:

geoarrow::read_geoparquet(system.file("example_parquet/nc_spherical-geoarrow.parquet", package = "geoarrow"), handler = s2::s2_geography_writer())
#> Error in UseMethod("wk_set_crs"): no applicable method for 'wk_set_crs' applied to an object of class "c('s2_geography', 's2_xptr')"